Method of making a thermoformable article having uniform distribution of coloring and mineral filler before and after thermoforming

ABSTRACT

The present invention is drawn to a composition and thermoformable sheets and articles made therefrom. In the present invention, ranges of chain-transfer agents, thixotropic agents, and mineral filler content are balanced to minimize migration or maldistribution of coloring matter and mineral filler during curing of methyl methacrylate in a syrup and also during subsequent heating and deformation in thermoforming, to achieve constancy of impact resistance and improve stability of patterns even in deformed portions of formed sheets.

RELATED APPLICATION

This application is a divisional of our application, Ser. No. 09/266,185filed Mar. 10, 1999, U.S. Pat. No. 6,177,499, which is a divisional ofSer. No. 08/888,958 filed Jul. 7, 1997, U.S. Pat. No. 5,985,972, whichis a continuation-in-part of our application of the same title, Ser. No.08/740,830, filed Nov. 4, 1996, now abandoned, which is acontinuation-in-part of Ser. No. 08/720,164, filed Sep. 25, 1996, U.S.Pat. No. 5,705,552, which is a continuation-in-part of Ser. No.08/620,510, filed Mar. 22, 1996, U.S. Pat. No. 5,567,745, which is adivisional of Ser. No. 08/392,650, filed Feb. 23, 1995, U.S. Pat. No.5,521,243, which is a continuation-in-part of Ser. No. 08/157,253, filedNov. 26, 1993 now abandoned.

TECHNICAL FIELD

This invention relates to the manufacture of acrylic sheets or slabs,that is sheets or slab of polymethylmethacrylate (“PMMA”), of the typeusable in or designed for architectural uses such as kitchen countertopsand more complex shapes. The sheets or slabs contain significant amountsof flame retardant minerals, typically alumina trihydrate, and almostalways have colorants in them, frequently in imitation of naturalmaterials such as onyx, marble or similar synthetic appearing solidcolor or patterned types having no visibly distinguishable particles.This invention describes a sheet that can be heated and bent at a sharp90° angle and/or that can be heated and vacuum formed into shapes likesinks and bowls without a significant aesthetic sacrifice. In addition,the sheets or slabs of this invention display specific physical andother properties, like low flammability and minimal color changes afterthermoforming, the uniform distribution of flame retardant significantlyimproves the consistency of impact resistance.

BACKGROUND OF THE INVENTION

Sheets and slabs of synthetic mineral appearing material are nowcommonly used as kitchen countertops and interior and exteriordecorative coverings of all kinds for buildings such as banks, airterminals, stores, and the like. Such applications frequently requirethat the material be fabricated to fit custom designed areas, requiringin turn that the slabs or sheets be butted together or otherwise joinedin ways that juxtapose a cross section with a normal surface at 90°.

The fabrication process requires extensive time and specially trainedcraftsmen to be completed successfully, since special tools andprocedures are necessary. If a shaped, one piece part of continuous ormonolithic material is desired, such a part can only be produced bycasting it in a mold cavity under special conditions. In addition to thehigh costs of such a process and for the installation of the parts(fitting, gluing it in place to a flat sheet, and/or finishing, forexample,) there are often color differences between the cast bowl, forexample, and the flat slab of the same material.

The sheet (the terms “sheet” and “slab” will be used interchangeablyherein) of our invention can provide a relatively complex finished partby a simple thermoforming operation—that is, the sheet is heated andthen pulled by vacuum into a concave cavity (or convex) mold, where itis allowed to cool, to retain its new shape. Such a mold can be shapedas a vanity top, with one 90° back splash wall, with a front end bullnose of 1.0 inch radius and a vanity type bowl. After forming, coolingand trimming, the part can be installed directly in place, withoutadditional fabrication required.

Only one contemporary commercial product (Corian® by DuPont) is said tobe capable of being heat bent. However, its performance is not suitable,for example, to make 90° angle back splash wall, since the minimumradius of curvature specified by the Corian® literature of which we areaware is 3.0 inches.

So far as we are aware, the use of alumina trihydrate inpolymethylmethacrylate (“PMMA”) articles was first proposed by Stevenset al in U.S. Pat. No. 3,563,939 (col. 4, lines 28-29) and Duggins inCanadian Patent 916,337. Its flame retardant properties are now wellknown and accepted, and alumina trihydrate (“ATH”) is now widely used asa filler in various resinuous products. Somewhat more detail for theconstruction of synthetic mineral products is provided by Duggins inU.S. Pat. No. 3,847,865; crosslinking agents are mentioned, for example.Also proposed are mold release agents, and viscosity reducers such asaliphatic acids.

Buser et al, in U.S. Pat. Nos. 4,085,246 and 4,159,301 address theproblem of the settling rates of various particles used in making asimulated granite having a matrix of polymerizable methyl methacrylate(“MMA”) having PMMA dissolved in it. See column 7, lines 42-62 of the'301 patent. They use the PMMA to adjust viscosity, which in turncontrols the settling rates of the larger particles—see the Examples,particularly Example 5 of U.S. Pat. No. 4,159,301, lines 31-34. Theyalso use chain-transfer agents as accelerators for thepolymerization—col. 8, lines 58-68 of the same patent.

Uniformity of color is mentioned as a goal in Gavin et al U.S. Pat. No.4,413,089, wherein iron oxide pigment of 10 microns or less is uniformlydistributed in a syrup of MMA/PMMA which is then cured; prolongedstorage of the syrup is not recommended (col. 2, lines 50-64).

In addition to meeting the above-described challenges, a materialdestined for use as a kitchen countertop, for example, should have asurface which is easily repairable and restored to its originalappearance, such as by sanding and polishing, be protected againstflammability, and have good temperature resistance in spite of beingthermoformable.

The prior art has more or less neglected the goal of thermoformabilityor thermobending of solid surface sheets, since the prior art productswere generally designed for reproducing the look of flat, natural,mineral based sheets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a more or less hypothetical illustration of a prior artbending of a sheet of “Corian®” one-half inch thick.

FIG. 1B is a similar idealized illustration of the bending of a sheet ofthe present invention.

SUMMARY OF THE INVENTION

The present invention addresses the making of mineral filled PMMA sheetsthat:

can be heat bent at relatively sharp angles,

can be thermoformed into shaped articles without losing the uniformappearance without losing the uniform appearance and properties of thetop surface,

can be thermoformed by vacuum into a single-profile mold, concave orconvex, and do not require two matching molds,

have only minor and tolerable color changes across the whole finishedpart, either less than Delta E=2.0 by Cielab or not easily discernibleby the human eye,

have a thermoforming temperature low enough to avoid any significantloss of water from ATH filler during thermoforming, as is often the casefor other thermoplastic materials,

have a Flame Spread Index, by the ASTM E-84 Tunnel Test, lower than 75and a Smoke Index of 350 or less,

have the same impact resistance, by a falling weight method, measuredfrom both the top side and the bottom side.

Our invention provides for the stability of the suspension of aluminatrihydrate, or other mineral filler, in a syrup of methyl methacrylatehaving polymethylmethacrylate dissolved in it by maintaining thefollowing ingredients within the indicated ranges (by weight):

Content of PMMA dissolved in MMA/other monomers: 0-30% weight,preferably 10-25%.

ATH in the entire composition: 20-60% by weight, preferably 25-40%.

Thixotropic agent (preferably fumed silica) in the monomer/syrupfraction of the mixture:

0.10-3.5% or as much as necessary to obtain a viscosity of 1,000-10,000centipoise (preferably about 2,000-5,000 centipoise) after mixing andmeasured by Brookfield™ Viscometer Model RVTDV-II, Spindle No. 2, 10RPM.

Crosslinking agent as % weight of the total monomers content: greaterthan 1%, and up to about 12%.

Chain-transfer agent as % weight of the total monomers content as itrelates to the amount of crosslinking agent “x”: when 1<x≦6,0.01≦y≦(1.07x+0.3), and when 6<x≦12, 7.0≧y≧(0.545x −3.23), when usingn-dodecyl mercaptan. A convenient way to compare the effects ofchain-transfer agents is to compare molecular weights obtained bypolymerizing MMA in the presence of the chain-transfer agent and theabsence of crosslinkers. The MW_(w) and MW_(n) should be similar to thatobtained by n-dodecyl mercaptan.

In addition to the above-identified ingredients, dyes and pigments maybe present, polymerization intiators will be necessary, and otherconventional ingredients may be used as are known in the art.

However, we do not employ particulates which are visibly distinguishablein the finished product. Most synthetic granites contain visiblydistinguishable particles of various compositions and colors rangingfrom about 150 to 500 microns—that is, they will pass through a sievehaving openings of 500 microns and be retained on one having openings of150 microns (although larger particles are not uncommon in the syntheticmineral art). We have found that our objective of even distribution ofparticles can be frustrated through the use of such larger particles ofvarious compositions, and accordingly, we restrict our particle size toparticles smaller than those which will be retained on a sieve havingopenings of 90 microns, and preferably smaller than those which will beretained on a sieve having openings of 60 microns. These specificationsfor particle size apply in our invention to particulates of anycomposition of function-mineral flame retardants such as ATH, forexample, or synthetic resin or other fillers.

The above-listed ingredients may be further described as follows:

PMMA as used herein is polymethylmethacrylate having a (weight average)molecular weight range of about 30,000 to about 600,000 having no crosslinked polymer chains, in order to remain soluble in MMA. It istypically made in situ by partial polymerization of methyl methacrylate,but can be pre-polymerized and dissolved in the MMA.

MMA is methyl methacrylate. The syrup is described herein as comprisingPMMA dissolved in monomers comprising at least about 60% MMA, andpreferably at least about 80% MMA, but of course the crosslinking agent,chain terminator, initiator, and thixotropic agent are also present inthe amounts indicated herein as well as variable amounts of dyes and/orpigments; in addition, amounts of other, optional, copolymerizablemonomers, notably butyl acrylate, may be present in the syrup as isknown in the art. We prefer to use a syrup which contains about 15% toabout 25% PMMA. References to syrup herein and to MMA should beunderstood possibly to include such additional materials.

Alumina trihydrate is well known in the art of synthetic mineralmanufacture. In the examples, we used it in a particulate size range ofabout 9 microns average, but the particulate size may vary widely. Asnoted above, the ATH as well as any other particles which arepotentially visually distinguishable (if large enough) in the finishedproduct should be able to pass through a sieve having openings of 90microns, and preferably will pass through a sieve having openings of 60microns, in order to assure that they will not be visuallydistinguishable. In quantity, the ATH may vary from about 20% to about60% weight (preferably 25% to 50%) of the finished product.

Our invention contemplates a solid surface material in which may be seenthe effects of the particulates no greater than 90 microns across, Ourmaterial is not simulative of granite, in that it is not coarse-grained,as granite is sometimes described. Rather, if the effects of theparticulates in our material can be discerned at all, it may bedescribed as substantially fine-grained (which we define specifically ashaving grains or particles less than 90 microns—that is, having noindividually visibly discernable particles greater than 90 microns). Weintend for the term “substantially fine-grained” to include materials inwhich no grains or particles are individually visibly discernable.

Any number of crosslinking agents, di-functional or tri-functional, maybe used. Examples for suitable crosslinkers are ethylene glycol dimethylacrylate, propylene dimethyl acrylate, polyethylene-glycoldimethacrylate, propylene dimethyl acrylate, polyethylene-gylcoldimethylacryalate, divinyl benzene, diallyl phthalate,1,3-butanediolmethacrylate, 1,4-butane ethylene glycol dimethacrylate orneopentyl glycol dimethacrylate as di-functional crosslinkers andtrimethylol propane trimethacrylate, triallyl cyanurate, pentaerythritoltetramethacrylate, allylmethacrylate, hydroxyethylmethacrylate orhydroxypropylmethacrylate as tri-functional crosslinkers. Most suitably,ethylene glycol dimethacrylate is preferred. The crosslinking agents aremaintained in concentrations of greater than 1.0. Preferably, 1.0 toabout 12.0 pph of di-functional crosslinkers based on the MMA in thesyrup, or, as a component of the finished product, based on thecrosslinked polymer. Most preferably, the content is 6.0≧x>1.0. Thecombination of crosslinking agent and chain termination in theappropriate amounts assures the appropriate polymeric network mostamenable to thermoformability.

Chain terminators or chain-transfer agents, such as octyl mercaptan,iso-dodecyl mercaptan, thiurams, dithiocarbarumates, dipentenedimercaptan, 2-mercapts ethanol, allyl mercapts-acetates, ethyleneglycol dimercapts-acetate, trimethylolethane trithioglycolate,pentaerythritol tetrathioglycolate, normally serve the function ofregulating the molecular weight of the polymerizing MMA, which in turnis known to affect the plastic behavior of polymerized mixture. Inaccordance with our method, chain terminators or chain-transfer agentsare used to regulate the length of the polymer chains and thus to obtainthe most suitable polymer matrix for thermoformability, as will be seenby the data in Example 3. They should be used in preferred amounts from0.01 to about 7.0 pph of the total monomers present when using n-dodecylmercaptan. Most preferably, 0.01≦y≦(1.07x+0.3).

While we may use a conventional thickening agent as well as athixotropic agent, the thixotropic agents we use are shown herein to beparticularly suited for present purposes. They appear to enhance theinertial tendency of a particle to remain stationary in the matrixsuspension. We prefer to use fumed silica. By fumed silica we identifythe product formed by the hydrolysis of silicon tetrachloride vapor in aflame of hydrogen and oxygen, to produce solid particles in the range7-30 millimicrons. Many different types of fumed silica are available.To conduct the bulk of our experimentation, we selected CAB-O-SIL® M5,which has a surface area of 200 sq.meter/gram. However, any conventionalfumed silica will have a beneficial effect in our invention.

The surface of fumed silica is hydrophilic since it has an abundance ofhydroxyl groups, which makes it capable of hydrogen bonding withsuitable molecules. Absorbed moisture in the silica or in the othercomponents has a gross effect on the final viscosity of suspensionscontaining fumed silica and normally it lowers it. The same effect isgiven by other substances which may be more or less capable ofdeveloping hydrogen bonding.

If the fumed silica and/or the ATH are dried to eliminate the absorbedmoisture, the final viscosity of the suspension will be higher than whenusing the commercial products directly from the containers in which theyare sold. Drying of the ATH above 200° F. may defeat its primary utilityas a flame retardant by depleting its water content.

In our preferred compositions, the amount of fumed silica is selected sothat the preferred viscosity is obtained, regardless of variations inthe other ingredients.

The preferred method of obtaining a desired viscosity is the following:

A. Mix all the ingredients (MMA, PMMA, ATH, pigments, other additives,catalysts, chain-transfer agent, and crosslinking agent) of theformulation except the fumed silica and measure the viscosity asindicated below. If necessary, adjust the MMA (monomer) content of thesyrup to obtain a viscosity of 800 to 1,500 centipoise.

B. Repeat step A including an amount of fumed silica and measure theviscosity.

C. Repeat step B to bring the viscosity to a level between 1,000 and10,000 centipoise, preferably between 2,000 and 5,000 centipoise.

As indicated previously, the stability of our syrup is consideredimportant, and this is especially so where the sheet or slab is formedin a continuous steel belt forming machine such as described inHellsund's U.S. Pat. No. 3,371,383 and Opel's U.S. Pat. No. 3,376,371,both of which are incorporated herein by reference in their entireties,as these references represent our preferred procedure. While the formingof sheets or slabs between two moving continuous steel belts is thepreferred procedure, it is important to realize that such machines arenecessarily prone to vibration and microadjustments which tend to resultin an almost unavoidable jostling of the particulates in the syrup; theconcentrations of crosslinker, chain terminator, fumed silica, and PMMAprepolymer are important in stabilizing the ATH and/or other solidscontributing to an evenly distributed fine-grained appearance.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1A, the recommended (DuPont Corian® Technical BulletinCTDC-110, October, 1987) minimum bending radius of three inches for aprior art one-half inch thick flat sheet is illustrated as the radius ofthe bend in the inside curve from vertical extension point A tohorizontal extension point B. Applying the simple formula C=IID, thecircumference of a hypothetical three-inch circle would be 18.8496inches, and,the quarter circle AB would measure 4.7124 inches. Applyingthe same formula to the outside curve for a sheet 0.5 inch thick, i.e.using a radius of 3.5, yields a quarter circle of 5.4953, a differenceof 16.6% from the inside curvature. Such a distortion will tend to causea flow of heated ingredients from the compressed inside curve to theexpanded outside, and lengthwise toward points A and B from the curvedportion. The flow of ingredients has a tendency to distort the visual ordecorative pattern; accordingly, the prior art has minimized thedisruptions of the material by using a relatively large radius for thecurvature, e.g. 3 inches.

FIG. 1B illustrates the achievable curvature of a sheet of the presentinvention, wherein the radius of the curve is one-half inch rather thanthe three inches of the section of FIG. 1A. In this case, thetheoretical circumference of the outside of the curved section CD is100% greater than that of the inside of the curve. It is readily seenthat by enabling such a forming ability, the present invention overcomesa more severe displacement of material in relatively less volume. Therelatively more severe displacement of material means a greaterpotential for distortion of the aesthetic pattern, but we avoid orneutralize such distortion and so achieve a continuity of patternheretofore not achievable under the stress of thermoforming. A test hasbeen devised to evaluate thermoformability, which is a primary object ofthe present invention. The test consists of clamping a flat testspecimen 4⅞″ square having the desired thickness onto a steel plate inwhich has been drilled a 3-inch diameter hole; then a polished stainlesssteel plunger having a one-inch radius is lowered at a rate of fiveinches per minute regardless of the resistance. The apparatus and sampleare heated prior to the test to the desired temperature. As the plungermoves, a load cell generates a signal representing the amount ofresistance in pounds, which may be recorded. At the moment the specimenruptures, the plunger is stopped and the distance it has traveled ismeasured. Averaging of tests from four specimens of each sample isrecommended. This test may be referred to herein as TP-0085.

EXAMPLE 1

A syrup was made by partial polymerization of MMA to obtain a viscosityof 3 Poise and a PMMA content about of 20% weight. Butyl Acrylate,CAB-O-SIL® M5) Aluminum Trihydrate (ATH) were added to the syrup underagitation. Their proportions are indicated below, together with thechemicals necessary to obtain a complete polymerization and a goodrelease from the call tasting plates:

TABLE 1 % Weight 1-3 syrup (80% MMA) 57.20 Butyl Acrylate 2.00 Cab-O-SilMS 0.53 ATH 39.92 Wetting Agents 0.35 Phr Pigment Paste As neededRelease Agents As needed Catalysts As needed Chain Transfer Agent SeeTable 2 Plasticizer See Table 2

The mixture of ingredients was first agitated under vacuum for 15minutes, to eliminate the dissolved gases and avoid bubbles in theresulting sheet. It was then used to fill a cell cast assembly, largeenough to produce a sheet of approximately 12×12×0.5 inches. The curingwas obtained by dipping the cell cast assembly into a 180° F. water bathfor one hour, followed by one hour of post cure in an air circulatedoven at 250° F. After cooling to room temperature, the cell castassembly was opened to remove the plastic sheet, and the physicaltesting described in the text was performed after conditioning at roomtemperature.

Table 2 shows the combinations of chain transfer agent and crosslinkerused and the test results.

The first group of data (PL-8, PL-11, PL-12, PL-14, PL-16, PL-19, PL-24,PL-25, PL-28 listed in Table 2 represents compositions that werethermoformed under vacuum to a much greater extent than compositionsfrom the known art. They were also thermally stable to provide formedshapes without defects.

The second group of data (PL-10, PL-13, PL-20, PL-23) representscompositions that failed the thermal stability test at 340° F. SamplePL-23 did not show a visible evidence of failing the blister test, butspecimens broken during the H.D.T. test and its stability were judged tobe not completely satisfactory.

The third group of data (PL-12, PL-22, PL-27) represents formulationsthat provide sheets with a borderline thermoformability. PL-22 and PL-27represent the demarcation between good formability above them and poorformability below them.

The V mold forming test referred to in Table 2 was developed todetermine what type of composition would yield a sheet that was animprovement over the prior art. For example, a 12 inch×12 inch Corian®sheet does not draw to any significant extent under vacuum.(Approximately 0.2 inches). Therefore, a sheet of the present inventionis an improvement over the prior art if the observed draw is greaterthan 0.2 inches.

TABLE 2 HDT⁽³⁾ TP-0085⁽⁴⁾ V Mold⁽⁵⁾ Blister⁽⁶⁾ Sample # x⁽¹⁾ y⁽¹⁾Plast.⁽²⁾ ° F. in./lbs. in. Temp ° F. PL-8 1.44 1.44 5.2/112 2.5 P PL-112.88 2.88 3.6/110 P PL-12 4.80 4.32 2.4/125 2.4 P PL-14 2.88 1.202.0/187 P PL-16 2.16 0.48 2.1/192 P PL-19 4.80 2.40 1.88 P PL-24 6.725.76 157 2.3/71 P PL-25 8.64 3.84 178 1.1/90 P PL-28 11.50 3.00 1710.7/38 P PL-10 1.44 2.40 6.9/23 F PL-13 2.88 3.60 Blistered F PL-20 4.805.28 2.7/55 F PL-23 7.68 6.72 F P PL-18 4.80 0.48 197 1.5/148 0.8 PPL-22 6.72 0.96 0.55 P PL-27 11.50 3.00 187 .54/68 P ⁽¹⁾Phr. ofcrosslinker x and chain transfer agent y per 100 parts of monomers inEx. 1 formulation. ⁽²⁾Phr. of plasticizer DINP (Di-isononyl phthalate)used. ⁽³⁾Heat distortion temperature, at 264 psi; per ASTM D-648.⁽⁴⁾Thermoforming test described in U.S. Pat. No. 5,521,243. Samples areheated for 40 minutes in an oven at 340° F. before the test isinitiated. ⁽⁵⁾V mold forming test. ⁽⁶⁾Aristech test method; visualobservations to determine if blisters were developed in a 4″ × 4″ sampleafter 40 minutes in an oven at 340° F.; P = indicates passing thes test;F indicates failure.

What is claimed is:
 1. A process for making a thermoformable articlecomprising the steps of: providing a syrup comprising methylmethacrylate, said syrup having dispersed within it x parts by weight ofa crosslinking agent and y parts by weight of a chain terminator, perhundred parts by weight of the methyl methacrylate, and wherein x isgreater than 1.0 and up to about 12 and when 1.0<x≦6.00.01≦y≦(1.07x+0.3),  and when 12>x>6.0 7.0≧y≧(0.545x−3.23);  andpolymerizing said syrup into said thermoformable article.
 2. The methodof claim 1, wherein 1.0≦x≦6.0.
 3. The method of claim 1, wherein6.0<x≦12.
 4. The method of claim 1 wherein said syrup further includes aplurality of solid particles, said plurality of solid particlescomprising about 20% to about 60% based on the weight of saidthermoformable article, of a mineral filler.
 5. The method of claim 4wherein said plurality of solid particles will pass through a sieve with90 micron openings.
 6. The method of claim 4, wherein the said pluralityof solid particles will pass through a sieve with 60 micron openings. 7.The method of claim 4, wherein said mineral filler is aluminatrihydrate.
 8. The method of claim 1 wherein said syrup further includesbutyl acrylate.
 9. The method of claim 1 wherein said syrup furtherincludes poly(methyl methacrylate).